Alkaline secondary batteries typified by a nickel-hydrogen battery and nonaqueous electrolyte secondary batteries typified by a lithium ion battery are widely used as power supplies for driving portable electronic equipment such as cell phones including smartphones, portable computers, PDAs, and portable music players. In addition, alkaline secondary batteries and the nonaqueous electrolyte secondary batteries are also widely used for power supplies for driving electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs) and in stationary storage battery systems for suppressing the variation in output power of photovoltaic generation, wind power generation, and the like, and for peak shifts in system power in order to store electric power during the night time and to use the electric power during daytime.
In particular, the batteries for EVs, HEVs, and PHEVs and for the stationary storage battery system are required to have high capacity and high output characteristics, and hence each battery is upsized and a number of batteries are connected in series or parallel when used. To address this, in these applications, prismatic secondary batteries are generally used from the viewpoint of space efficiency. A prismatic secondary battery that further needs physical strength commonly employs, as an outer body of the battery, a metal prismatic outer body having a mouth and a metal sealing plate for sealing up the mouth.
Such a prismatic secondary battery, for example, a prismatic nonaqueous electrolyte secondary battery, is produced as follows. For example, both faces of a positive electrode substrate made from, for example, a long sheet of aluminum foil, are coated with a positive electrode active material mixture containing a positive electrode active material to prepare a positive electrode sheet. Separately, both faces of a negative electrode substrate made from, for example, a long sheet of copper foil, are coated with a negative electrode active material mixture containing a negative electrode active material to prepare a negative electrode sheet.
Next, the positive electrode sheet and the negative electrode sheet are stacked interposing a separator made from, for example, a microporous polyethylene film therebetween, and the positive electrode sheet and the negative electrode sheet are spirally wound on a cylindrical winding core while insulating the positive electrode sheet and the negative electrode sheet from each other through the separator to prepare a cylindrical wound electrode assembly. Then, the cylindrical wound electrode assembly is pressed with a pressing machine to form a flat wound electrode assembly. Next, a positive electrode collector electrically connected to the positive electrode sheet is electrically connected to a positive electrode terminal that is insulated from a sealing plate, while a negative electrode collector electrically connected to the negative electrode sheet is electrically connected to a negative electrode terminal that is insulated from a sealing plate. Then, the flat wound electrode assembly is wrapped with an insulating sheet and stored in a metal prismatic outer body; a mouth portion of the prismatic outer body is sealed with a sealing plate; an electrolyte is poured from a electrolyte pour hole provided on the sealing plate; and finally the electrolyte pour hole is sealed to produce the prismatic nonaqueous electrolyte secondary battery.
When a plurality of such prismatic secondary batteries required to have high capacity and high output characteristics are combined to form a battery pack, for example, as shown in JP-A-2008-235149 and US Patent Publication No. 2008/299453 (A1), a resin spacer having a size smaller than the outer size of the battery may be interposed between the respective batteries, and the batteries may be pressurized until the battery has a thickness not larger than a required thickness (can thickness) for containment. Moreover, for example, as shown in JP-A-2009-032640 and US Patent Publication No. 2009/297940 (A1), a prismatic secondary battery includes a collector having a rib in a battery thickness direction in order to suppress the dispersion of spattered particles generated during resistance welding when the collector is resistance-welded to a position at which a plurality of positive electrode substrate exposed portions or a plurality of negative electrode substrate exposed portions are bundled. With the prismatic secondary battery using such a collector having the rib in the battery thickness direction, the rib shields spattered particles generated during resistance welding and the spattered particles are unlikely to enter into the battery. Therefore, a prismatic secondary battery with high reliability can be obtained.
In a prismatic secondary battery, a substrate constituting a positive electrode may have a thickness different from that of a substrate constituting a negative electrode. In a prismatic secondary battery required to have high capacity and high output characteristics, the stacking number of the positive electrode substrates is approximately the same as that of the negative electrode substrates. However, each stacking number is large, resulting in a large difference in the thickness between a bundled plurality of positive electrode substrate exposed portions and a bundled plurality of negative electrode substrate exposed portions. In this case, when a positive electrode collector and a negative electrode collector each have a rib with the same height, the distance between the leading end of the rib and the prismatic hollow outer body in the positive electrode side differs from that in the negative electrode side.
When a plurality of such prismatic secondary batteries in which the distance between the leading end of the rib and the prismatic hollow outer body in the positive electrode side differs from that in the negative electrode side are combined to form a battery pack, the batteries are not uniformly depressed in a process of applying pressure to the batteries until the battery has a thickness not larger than a required thickness for containment. Thus, the produced battery pack suffers from varied length and bending.
In the prismatic secondary battery disclosed in US Patent Publication No. 2009/297940 (A1), the leading end of each of the ribs provided to the electrode collectors breaks into an insulating sheet disposed between a flat electrode assembly and a prismatic outer body, but the height of the rib is designed so that the rib does not break through the insulating sheet. However, US2009/297940 (A1) does not disclose the ribs having different heights between the positive electrode side and the negative electrode side.